1. Field of the Invention
This invention relates to an exhaust emission control system for an internal combustion engine of a motor vehicle.
2. Description of Related Art
At present, an exhaust emission control system is used for detoxifying an exhaust gas emitted by a motor vehicle so as to discharge a nontoxious gas into air, and plays an important role for ecology.
One of such exhaust emission control systems is a three way catalyst which includes an oxidation catalyst and a reduction catalyst. When the air/fuel ratio is maintained in the narrow window including the stoichiometric ratio, the oxidation catalyst oxidizes carbon monoxide CO and hydro carbons HC, and the reduction catalyst reduces the nitrogen oxides NOx, so that the exhaust gas can be converted into a nontoxious gas.
To operate the engine more effectively at a reduced cost, it is considered to be advantageous to use a fuel mixture whose air/fuel ratio is above the stoichiometric ratio, i.e. "lean". A number of motor vehicles incorporated an engine operating on a lean air/fuel mixture have been developed up now. The three way catalyst can function effectively as the exhaust emission control system when the air/fuel ratio is the stoichiometric ratio. The three way catalyst however has a rather poor catalytic function when excess oxygen is present in the mixture (i.e. lean), and is difficult to satisfy the emission regulation.
To overcome this problem, Japanese Patent Laid-Open Publication No. Sho 60-125,250 exemplifies a catalyst which can reduce the nitrogen oxides NOx in a lean exhaust gas. This catalyst is called "lean NOx catalyst" hereinafter.
The lean NOx catalyst tends to age under a high temperature and requires hydro carbons while reducing NOx, which makes the catalyst disadvantageous to be installed in the motor vehicle.
The lean NOx catalyst cannot exhibit its sufficient NOx purifying ratio (.eta..sub.NOX) and cannot function effectively until the ratio of hydro carbons and carbon monoxide reaches a predetermined value in the exhaust gas as shown in FIG. 4 of the accompanying drawing figures. Therefore, when the lean NOx catalyst and the three three-way catalyst are disposed in an exhaust path, it is necessary to position the lean NOx catalyst upstream of the three way catalyst so as to keep sufficient hydro carbons.
To enhance purification of the exhaust gas as Soon as the engine is started, it is necessary to activate the catalyst as quickly as possible. For this purpose, a relatively small first catalyst (warm-up catalyst) is disposed near an exhaust port of the combustion chamber of the engine, and a second main catalyst is disposed downstream of the first catalyst. Thus, the first catalyst composed of the three way catalyst is early heated to be active, so that the exhaust gas can be quickly and effectively purified after the engine is started.
The stricter the emission regulation, the more demand will be placed on the warm-up catalysts in the future. When the three way catalyst which requires hydro carbons to purify NOx is used as the worm-up catalyst without any modification, the lean NOx catalyst in the main catalyst cannot function normally because of an insufficient amount of hydro carbons. On the other hand, since the lean NOx catalyst is abruptly aged under a high temperature, it cannot be applied as the warm-up catalyst as it is.
Furthermore, the lean NOx catalyst tends to age extensively when the engine is working with a rich mixture in which excess fuel is present, or a mixture having the stoichiometric ratio.